My Cover

With regard to your last sentence there is no question of a caravan being left on the hard shoulder of a motorway. It just doesn't happen and the police and highways agency would not allow it to happen. 

The broken down car and caravan will be taken to a safe place or a garage where a repair can be undertaken. What happens after that if a repair cannot be managed depends on which of the three levels of Green Flag cover one has, or whether it is Mayday cover which Green Flag operates. All those four levels of cover differ but the policy documents spell out the options. 

There may well  be differing cover levels between Green Flag and Mayday.

I am not suggesting that Green Flag have ever left a caravan on the hard shoulder, its just that they are under no obligation to recover the van. It is after all the travelers responsibility.

The policy documents are, it seems deliberately vague. They clearly do not spell out the options.

I'd be interested to know if there is a difference and if so, what those differences are.

There are websites and policy documents on line for Green Flag's three levels of cover and also for CMC Mayday which Green Flag operates. There's no short cut to reading them carefully. 

For instance Green Flag's top level of cover (Europlus) includes continental breakdown cover, but CMC Mayday is strictly UK only.